The present invention relates to visual displays, and more particularly to addressable, reusable, paper-like visual displays, and to gyricon or twisting-ball displays.
Since ancient times, paper has been a preferred medium for the presentation and display of text and images. The advantages of paper as a display medium are evident. For example, it is lightweight, thin, portable, flexible, foldable, high-contrast, low-cost, relatively permanent, and readily configured into a myriad of shapes. It can maintain its displayed image without using any electricity. Paper can be read in ambient light and can be written or marked upon with a pen, pencil, paintbrush, or any number of other implements, including a computer printer.
Unfortunately, paper is not well suited for real-time display purposes. Real-time imagery from computer, video, or other sources cannot be displayed directly with paper, but must be displayed by other means, such as by a cathode-ray tube (CRT) display or a liquid-crystal display (LCD). Typically, real-time display media lack many of the desirable qualities of paper, such as physical flexibility and stable retention of the displayed image in the absence of an electric power source.
Attempts have been made to combine the desirable qualities of paper with those of real-time display media in order to create something that offers the best of both worlds. That something can be called electric paper.
Like ordinary paper, electric paper preferably can be written and erased, can be read in ambient light, and can retain imposed information in the absence of an electric field or other external retaining force. Also like ordinary paper, electric paper preferably can be made in the form of a lightweight, flexible, durable sheet that can be folded or rolled into tubular form about any axis and conveniently placed into a shirt or coat pocket, and then later retrieved, re-straightened, and read substantially without loss of information. Yet unlike ordinary paper, electric paper preferably can be used to display full-motion and other real-time imagery as well as still images and text. Thus it is adaptable for use in a computer system display screen or a television.
The gyricon, also called the twisting-ball display, rotary ball display, particle display, dipolar particle light valve, etc., offers a technology for making a form of electric paper. Briefly, a gyricon is an addressable display made up of a multiplicity of optically anisotropic bas, each of which can be selectively rotated to present a desired face to an observer. For example, a gyricon can incorporate balls each having two distinct hemispheres, one black and the other white, with each hemisphere having a distinct electrical characteristic (e.g., zeta potential with respect to a dielectric fluid) so that the balls are electrically as well as optically anisotropic. The black-and-white balls are embedded in a sheet of optically transparent material, such as an elastomer layer, that contains a multiplicity of spheroidal cavities and is permeated by a transparent dielectric fluid, such as a plasticizer. The fluid-filled cavities accomodate the balls, one ball per cavity, so as to prevent the balls from migrating within the sheet. A ball can be selectively rotated within its respective fluid-filled cavity, for example by application of an electric field, so as to present either the black or the white hemisphere to an observer viewing the surface of the sheet. Thus, by application of an electric field addressable in two dimensions (as by a matrix addressing scheme), the black and white sides of the balls can be caused to appear as the image elements (e.g., pixels or subpixels) of a displayed image.
The gyricon is described further in the patents incorporated by reference hereinabove. In particular, U.S. Pat. No. 5,389,945 (Sheridon, "Writing System Including Paper-Like Digitally Addressed Media and Addressing Device Therefor") shows that gyricon displays can be made that have many of the desirable qualities of paper, such as flexibility and stable retention of a displayed image in the absence of power, not found in CRTs, LCDs, or other conventional display media. Gyricon displays can also be made that are not paper-like, for example, in the form of rigid display screens for flat-panel displays.
Although the gyricon represents an important step toward the goal of electric paper, there is still a long way to go. For example, a gyricon constructed of black-and-white balls cannot provide a multicolor image. As another example, a gyricon designed to operate in ambient reflected light cannot provide a projective or transmissive display. What is needed is an advanced gyricon technology that can provide a more full range of display capabilities while preserving paper-like advantages.
GOODRICH (U.S. Pat. No. 4,261,653, "Light Valve Including Dipolar Particle Construction and Method of Manufacture") discloses a light valve based on a spherical ball that can be rotated between a first orientation and a second orientation through the application of oscillating electric fields of two different frequencies. Goodrich's spherical ball is made up of a light-absorptive or light-reflective central segment surrounded by transparent intermediate and outer segments. In the first orientation of the ball, the central segment is transverse to the direction of incident light and so blocks the passage of light. In the second orientation of the ball, the central segment is aligned with the direction of incident light and so admits the passage of light, which passes through the transparent portions of the ball. Rotation between the first and second orientations is accomplished by changing the frequency of an applied oscillating electric field from a high frequency (e.g., 10,000 Hz) to a low frequency (e.g., 100 Hz), and taking advantage of the frequency-dependent dielectric characteristics of the intermediate segments and the frequency-insensitive dielectric characteristics of the outer segments. When the frequency of the applied field is high, the dielectric constant of the intermediate segments becomes less than that of the outer segments, and the induced charge in the intermediate segments causes the ball to orient in the first orientation. When the frequency of the applied field is low, the dielectric constant of the intermediate segments becomes greater than that of the outer segments, and the induced charge in the intermediate segments causes the ball to orient in the second orientation.
Goodrich's frequency-dependent addressing scheme requires specialized, possibly cumbersome addressing electronics and an AC voltage source capable of delivering high frequencies (e.g., RF frequencies). Goodrich's light valve balls (although said by Goodrich to be "dipolar") would not be responsive to a change in orientation of the electric field vector of a steady-state, nonoscillating electric field. Thus Goodrich's overall approach does not appear to be readily adaptable for use with electric fields produced from a simple DC voltage source without transformation to high-frequency AC.